Cinnamic reaction with benzene

Oxidative coupling of arenes with alkenes consumes a stoichiometric amount of Pd(II). Efficient catalytic reaction of cinnamate (165) with benzene to afford 166 was carried out using BQ and f-butyl hydroperoxide as oxidant in AcOH [70]. The coupling proceeds smoothly in the presence of catalytic amounts of Pd(OAc)2 and molybdovanadophosphoric acid (HPMoV) under oxygen (1 atm) in AcOH [71]. 

Scheme 2.18 Proposed mechanism for the reaction of cinnamic acid with benzene.

Table 2.22 Products and relative yields from the reactions of poro-substituted cinnamic acids with benzene and TfOH (100 equiv 12 h at 25°C) ...

The first reactions concerned (Simons and Archer, 27) alkylation of benzene with propylene to form isopropylbenzene, with isobutene to form f-butylbenzene and di-f-butylbenzene, and trimethylethylene to form amylbenzene. Later on (Simons and Archer, 28) studied these and other reactions in more detail and showed that high yields could be obtained and that the product was not contaminated with tars or other obnoxious impurities. It was shown that the products obtained with trimethylethylene were mono- and di-f-amylbenzene, that phenyl-pentane resulted from the use of pentene-2, and that cyclohexene produced cyclohexylbenzene. Cinnamic acid reacted with benzene (Simons and Archer, 29) to form /3-phenylpropionic acid and allyl benzene reacted with benzene to form 1,2-diphenylpropane. It is interesting to note that although allyl alcohol reacted with benzene to form 1,2-diphenylpropane, the intermediate in the reaction, allylbenzene, was isolated and identified. This shows that in this case the hydroxyl reacted at a more rapid rate than the double bond. Both di- and triisobutylene reacted with phenol (Simons and Archer, 30) at 0°, when using hydrogen fluoride containing only relatively small quantities of water, to form f-butyl-benzene, but diisobutylene with 70% hydrogen fluoride produced p-f-octylphenol. Cyclohexene reacted with toluene to form cyclohexyl-toluene and octene-1 rapidly reacted with toluene to form 2-octyltoluene (Simons and Basler, 31). 

Klumpp and co-workers362 have performed a detailed study of the acylation reactions of benzene with a variety of cinnamic acid derivatives in triflic acid. Cinnamic acids with alkyl or weakly electron-withdrawing groups (F, Br) on the phenyl ring give the corresponding substituted indanones as a result of a two-step... 

Heteropolyoxametalates are often used in combination with palladium salts as catalysts in oxidation processes using dioxygen as the oxidant. Indeed, the oxidative coupling reaction of benzenes with alkenes was also successfully achieved by use of the Pd(OAc)2/molybdovanadophosphoric acid (HPMoV)/02 system [14a]. For example, reaction of benzene with ethyl acrylate using this catalytic system in acetic acid afforded ethyl cinnamate as a major product in satisfactory yield. Typically, the reaction is conducted in acetic acid at 90 °C under 1 bar of 02. After 6 h the TON is 15. This number was recently improved to 121 [14b]. 

For the compounds obeying the Hammett equation, the distribution pattern for the OH attack can be estimated by assuming that the position on the ring with the maximum C7 value has the least probability, while the one with the minimum has the greatest probability. In our recent work > on cinnamate derivatives where the addition of the OH radical to the olefmic double bond is an additional pathway, a value of-0.3 was estimated from the Hammett plot (Fig. 5). This value is in agreement with those reported earlier for several substituted benzenes (p" = -0.52 to -0.4). However, the Hammett treatment for reactions with rate constants close to diffusion-controlled ones may not be satisfactory due to limited variation in kinetic data and large experimental uncertainty. In contrast, better correlation is expected in the corresponding gas phase reactions. 

Pans-2-Iodocyclohexyl acetate can be isolated in essentially quantitative yield from the reaction of thallium(I) acetate, iodine, and cyclohexene in a 1 1 1 molar ratio in refluxing chloroform. lodo acetates from a representative series of alkenes including cyclohexene have been similarly prepared in 80-98% yield in glacial acetic acid which was not dried as described in this procedure. The corresponding iodo benzoates are obtained in comparable yields from reaction with thallium(I) benzoate and iodine in benzene. The deactivated olefin methyl cinnamate did not react under these conditions, and o-allylphenol underwent ring iodination to... 

Condensation. Cinnamic acid condenses with benzene in zeolites, but the reaction pattern depends on the type of the zeolite used/... 

The addition of benzoquinone led to the development of more active reoxidation systems. A substoichiometric amount is sufficient when rerr-bntyl hydroperoxide is present as a cooxidant, leading to TONs of up to 280 in the reaction of benzene with ethyl cinnamate (Figure 4.6) [ 14]. Under these conditions, the reaction was found to be applicable to various arenes in combination with several electron-deficient alkenes. Hydrogen peroxide was also found to be a suitable co-oxidant, although the resulting environmentally benign protocol, in which water is formed as the only by-product, gave lower turnover (TON = 35). 

Ruthenium has a rich chemistry of hydroarylation reactions [22], but it has also been used successfully by Milstein and coworkers [23] as a catalyst for oxidative couplings of the Fujiwara-Moritani type (Figure 4.12). Under an atmosphere of carbon monoxide (6 bar), various ruthenium precursors effectively promoted the reaction of acrylates (e.g., 4g) with benzene (2a) to give a 1 1 ratio of the (E)-cinnamate 5i and methyl propionate 12, rather than the expected hydroarylation product methyl 3-phenylpropionate. Added oxygen (2 bar) could partly take over the role of the reoxidant from the alkene, resulting in an increase in the incorporahon of the alkene into the cinnamate product, giving a ratio of up to 3 1 of the arylated to the reduced acrylate. 

Much more conveniently, even a,)S-unsaturated esters can he transformed into a,)S-unsaturated alcohols by very careful treatment with lithium aluminum hydride [1073], sodium bis(2-methoxyethoxy)aluminum hydride [544] or diiso-butylalane [1151] (Procedure 18, p. 208). An excess of the reducing agent must be avoided. Therefore the inverse technique (addition of the hydride to the ester) is used and the reaction is usually carried out at low temperature. In hydrocarbons as solvents the reduction does not proceed further even at elevated temperatures. Methyl cinnamate was converted to cinnamyl alcohol in 73% yield when an equimolar amount of the ester was added to a suspension of lithium aluminum hydride in benzene and the mixture was heated at 59-60° for 14.5 hours [1073]. Ethyl cinnamate gave 75.5% yield of cinnamyl alcohol on inverse treatment with 1.1 mol of sodium bis(2-methoxy-ethoxy)aluminum hydride at 15-20° for 45 minutes [544]. 

Evans and co-workers were the first to report that 4,4 -disubstituted bisoxazolines 29 are excellent chiral ligands for enantioselective aziridination (Scheme 6B.30) [74,75]. Aryl-substi-tuted olefins, especially cinnamate esters, are good substrates for this aziridination. The best reaction conditions, however, vary with the substrates used. For the reactions of cinnamate esters, bisoxazoline 29a and benzene are the ligand and solvent of choice. Under these conditions, enantioselectivity up to 97% ee is observed. For the aziridination of styrene, bisoxazoline 29b and acetonitrile are the appropriate ligand and solvent. 

Polymerizations have been carried out under nitrogen in THF or benzene with azobisisobutyronitrile (3% weight) as initiator (Table II). The results are different according to the nature of the cinnamic double bond substituents. Monomers 1 and 2 and corresponding polymers can indeed lose one acid function during polymerizations carried out at reflux temperature. This decarboxylation is in relation with the yield of gel product and molecular weight. Therefore monomers 3 and 4 for which no decarboxylation could occur, have been polymerized without gel formation with high yields. Likewise decarboxylation reaction seems to induce transfert reaction. 

Phosphine-containing crown ethers, combined with [Rh(l,5-cycloocta-diene)Cl]2, promote hydrogenation of alkali-metal cinnamates [194]. It has been also demonstrated that ( 7r-C3H5PdCl)2, in the presence of such crown ethers, catalyzes the reaction of allyl bromide with powdered Nal or KI in benzene [194]. 

Intramolecular Additions - The truxinic acid derivative (23) is formed on irradiation of (24) in methanol using a Pyrex filter. The reaction is quantitative and has a quantum yield of 0.55. Similar reactivity is observed when crystals of (24) are irradiated. The use of a benzene ring as a constraint has provided a method for the formation of a single stereoisomer in high yield this is seen best of all with the yield this is cinnamate ester (25). Irradiation of (25) for 21 h in methylene chloride solution affords an 88% yield of (26). A less efficient reaction... 

With this ligand, aziridination of chromene 7.40 (X = O, R = Me, R = 4-CN) takes place at -78°C with an ee superior to 98% and aziridination of 7.40 (X = CH2, R=R =H) takes place with 87% ee. From other olefins, disappointing results are observed. Evans and coworkers recommended bis-oxazolines 3.28 (R = Ph, R = H, R" = Me) as copper ligands for asymmetric aziridination of cinnamic esters [965] (Figure 7.35). These reactions take place in benzene at room temperature in the presence of molecular sieves. However, other olefinic substrates again do not give high selectivities. 

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